I think it comes from the ?medieval belief that emotions were felt in or based in different parts of the body, eg love, heart,fear, stomach and the spleen was supposed to be the seat of anger.
Iggy, If you click on http://en.wikipedia.org/wiki/The_four_humours
the link will take you to an informative website about the bodily 'humours', which were believed in as long ago as by the ancient Greeks. The spleen was originally associated particularly with a melancholy temperament, but Shakespeare seems to have been the first to associate it with rage, which is what we still take it to mean today.